BACKGROUND: Donors are deferred for multiple reasons. Losses related to disease marker rates are well established. Donor and donation losses for other reasons, however, have not been extensively quantified. STUDY DESIGN AND METHODS: To quantify these losses, three data sets from the Blood Centers of the Pacific were combined, permitting detailed analysis of year 2000 allogeneic whole-blood donations. RESULTS: During 2000, 13.6 percent of 116,165 persons who presented for donation were deferred at presentation. Short-term deferral accounted for 68.5 percent (hematocrit was most common at 60%); long-term deferral accounted for 21 percent (travel to a malarial area and tattoo or other nonintravenous drug use needle exposure were most common at 59 and 29%, respectively); and multiple-year or permanent deferral accounted for 10.5 percent (UK travel [variant Creutzfeldt-Jakob disease] risk and emigration from a malarial area were most common at 38 and 11%, respectively). Disease-marker-reactive donations represented 0.9 percent of donor outcomes. The prevalence of deferral and also miscollection (under- and overweight units) varied by age, sex, and first-time versus repeat donor status. Overall, miscollection led to a loss of 3.8 percent of 100,141 collections, ranging from 1.9 percent in repeat male donors 40 to 54 years of age to 10.7 percent in first-time female donors 16 to 24 years of age. CONCLUSION: Loss of units from both first-time and repeat donors due to temporary deferral and loss of units from miscollection are more common events than losses due to disease marker testing. Some of these losses may be avoidable and could increase the blood supply without having to recruit new donors.